Just some ideas for post-threshold mutations unique to Fish.
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Schooling - Grants the player a variety of passive boosts based on the number of nearby allies. For each visible follower within 5 tiles: +0.5 dodge bonus, +2.5 morale, +2% speed. Subject to balancing.
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Fins - Grants a large boost to swim speed and +10% overall speed in water.
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Razorfins (requires Fins) - Growths extending from the sides of the player’s arms. Can be used for fast, moderately damaging cutting attacks when unarmed. The player can still wear clothing on the arms, however this renders the razorfins unusable.
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Lateral Line - Sensory organ providing a +3 dodge bonus in water, +1 out of water.
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Luminescent Organ - Togglable utility mutation that allows the player to generate enough light to read and craft by. Might also provide some utility when fishing, such as by increasing speed and success rate when fishing at night.
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Warning Colors (requires Spots) - Highly visible mutation that discourages natural creatures from attacking the player. Could also be used in the Spider mutation category.
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Electroreception - Allows the player to detect anything with electrical impulses in its body. Includes all natural living creatures, robots, and CBM zombies (shockers, shocker brutes, technicians, scientists, bio-operators). May or may not include certain Nether creatures.
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Bioelectric (requires Electroreception) - Provides shockback defense at the expense of 1 hunger each time it is activated.
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Shark Jaw (requires Fangs, cancels Glass Jaw) - Provides a more powerful opportunistic bite attack.
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Dermal Denticles (requires Sleek Scales) - Provides resistance to infectionand parasites, and does a small amount of cutting damage to unarmed attackers.
I would also suggest adding some more pre-existing mutations to the Fish category, such as
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Quills and Venomous - Reminiscent of a lionfish or puffer fish. Venomous on its own also synergizes with Razorfins and gives Warning Colors some merit.
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Strong - For the strength demonstrated by fish leaping out of the water or slapping fishermen really hard.
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The Apex Predator series - sharks, piranhas, and barracudas.